1. Field of the Invention
This invention relates to a television camera apparatus having a negative-to-positive inverting function.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows the circuit arrangement of the conventional color TV camera apparatus of the kind having a negative-to-positive inverting function. Referring to FIG. 1, image sensing light 2 coming from a shooting object 1 is guided to an image sensor 5 via a lens 3 and an iris 4. The image sensor 5 photo-electrically converts the image sensing light 2 coming from the object 1 and produces color signals for colors R (red), G (green) and B (blue). Each of the color signals R, G and B are supplied to a signal processing circuit 6. The circuit 6 then performs suitable signal processing actions such as white balance adjustment, gamma correction, etc. These processes are followed by a matrix process. After the matrix process, the signals are produced in the form of a luminance signal Y and color-difference signals R-Y and B-Y. The luminance signal Y and the color-difference signals R-Y and B-Y are inverted from negative to positive by negative-to-positive inverters 7a, 7b and 7c. The inverted luminance signal Y and the inverted color-difference signals R-Y and B-Y are supplied to an encoder 8. These inverted signals are then converted into a video signal, which is output to the outside.
The above-stated inverting process is mathematically expressed by the following formula: ##EQU1##
In other words, the inversion of the luminance signal comprises obtaining a difference between a reference level signal A and the luminance signal Y. The inversion of each of the color-difference signals comprises a process of inverting the polarity thereof.
However, in the conventional color TV camera apparatus of the above-stated kind, the relation of the level of luminance to the degree of color saturation is reversed when the open position (aperture value) of a light-quantity adjusting iris 4 is shifted from an apposite light-quantity position in the negative-to-positive inverted state. This has presented a problem, because the natural state of the object image is reversed to degrade its colors and thus hardly gives any adequate image.
More specifically, in the event of negative-to-positive inversion, the degree of color saturation becomes higher when the luminance level of the video signal output becomes lower as a whole. It becomes lower when the luminance level becomes higher. Thus, this acts contrary to the natural state. For example, when the luminance level in the video signal output becomes lower, if the iris 4 is opened to make the luminance signal Y larger before inversion, the luminance signal Y after inversion becomes smaller as apparent from Formula (I) above. Further, since the iris 4 is opened in this instance, the absolute values .vertline.R-Y.vertline. and .vertline.B-Y.vertline. and of the color-difference signals R-Y and B-Y become larger than the values obtained when the light quantity is apposite. Therefore, the absolute values .vertline.R-Y.vertline. and .vertline.B-Y.vertline. of the color-difference signals R-Y and B-Y after inversion also become larger as apparent from the following formula: ##EQU2## This means that the state of color is good at a part where the level of the input signal is high, i.e., where the level of the inverted signal is low, and is bad at a part where the level of the input signal is low, i.e., where the level of the inverted signal is high, even in the case of adequate incident light quantity. Thus, there is obtained a state which is reverse to the natural state. FIGS. 2(a) and 2(b) illustrate this state. FIG. 2(a) roughly shows the wave form of the video signal output obtained in a normal state without negative-to-positive inversion. The coloring is obtained in a relatively small amount of saturation a1 (the amplitude component of a carrier signal) at a part where the luminance level A1 is low. The coloring is obtained in a relatively large amount of saturation b1 at a part where the luminance level B1 is high. When this state is inverted, a wave form as shown in FIG. 2(b) is obtained. As shown, at a part where a luminance level A2 is high, there is a signal of a relatively small amount of saturation a2. At a part where a luminance level B2 is low, there is a signal of a relatively large amount of saturation b2.
Further, the conventional color TV camera apparatus of the above-stated kind has presented another problem, which is as follows: In a case where the object 1 is a negative photo film, the luminance level is low and the degree of color saturation is small. The whole picture is dark with very thin colors. In that case, it is hardly possible to obtain an adequate image.
As is well known, the negative photo film has such a characteristic that the quantity of light incident upon the film surface is in a non-linear relation to the blackening degree of the film. This is called a gamma characteristic. It varies to a less degree at a part where the incident light is in a larger quantity. In other words, an image is printed on the negative film in a state of being compressed at a high luminance level part. Therefore, an output produced by photo-electrically converting the light coming from the negative film by the image sensor is also obtained in a state of being compressed at its part where the luminance of the object is high before shooting. This is shown in FIGS. 3(a) to 3(e), which roughly show a case where a gray scale chart is employed as an object to be photographed. FIG. 3(a) shows the distribution of luminance of the chart obtained before shooting. The axis of abscissa shows the spatial position of the chart and the axis of ordinate the luminance level. Further, in FIG. 3(a), the axis of abscissa corresponds to the horizontal (H) direction which is usually used for the TV camera signal and is expressed as if parts in the vertical (V) direction of the TV camera signal are added together therewith. FIG. 3(b) shows an output signal (the signal G, for example) of the image sensor. This corresponds to the distribution of blackening degree of the negative film (object 1). In FIG. 3(b), what is shown in FIG. 3(a) is shown upside-down. Since the object is a negative film and because of the above-stated characteristic, the lower part of FIG. 3(b) (corresponding to the upper high luminance part of FIG. 3(a)) is salient in a non-linear state. Further, in FIG. 3(b), a broken line represents the output characteristic of the image sensor obtained when the blackening degree characteristic of the negative film is in a linear state. The signal which is thus obtained is supplied to the signal processing circuit 6 to undergo the gamma correction process (to obtain a gamma value of 0.45 by the gamma process within the TV camera) after white balance adjustment, and, then, the wave form of the signal becomes as shown in FIG. 3(c). The high level part of FIG. 3(b) is compressed in this case. In FIG. 3(c), a part indicated by a broken line represents a level obtained before the gamma correction process. The signals R and B are likewise processed. After that, the signals Y, R-Y and B-Y are formed through a matrix process. Since this is a so-called linear matrix process, the characteristics of these signals obtained before the matrix process are reflected as they are in the outputs of the matrix.
More specifically, as shown in FIG. 3(d), the luminance signal Y becomes a signal which is inverted and compressed as a whole as compared with the original level of the object. The levels of the color-difference signals are also transformed into a compressed shape.
Therefore, when the signals Y, R-Y and B-Y are processed through the inverters 7a, 7b and 7c, the normal output levels become lower as they are inverted with their levels having been compressed. As a result, the image thus obtained appears dark as a whole on the picture screen and in thin colors. FIG. 3(e) shows the luminance signal Y as in a state after the inversion.
Further, with respect to an image to be obtained from a negative film or the like by the conventional camera apparatus, each of the characteristics relative to, for example, the gains for gamma, black-and-white, clipping, amplification, etc., of the color-video-camera signal processing circuit is generally set on the basis of a positive image of an ordinary object. Generally, compared with an ordinary object, an object on a negative film has a less degree of difference between brightness and darkness in luminance. Further, when the object on the negative film is displayed on a monitor, the video signal output level becomes low to result in a dark image. If the iris is opened wider for brightening the image, linearity tends to be lost. Therefore, it has been hardly possible to obtain a luminance level with an adequate linearity.